Climate change has already exerted far-reaching effects

Thanks in large part to human activity, including the burning of fossil fuels and deforestation, carbon dioxide (CO2) levels in the atmosphere are rising. The impacts of this change in Earth’s atmosphere are likely to increase significantly over coming decades, but they are already apparent.

Much of the evidence is summarized in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report, published in 2007.[[Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. Link.]] Like earlier versions, this report is a synthesis of hundreds of individual peer-reviewed scientific papers on the causes and effects of climate change.

One direct result of the increase in anthropogenic (i.e., human-caused) CO2 concentrations is a rise in global temperatures, due to an enhanced greenhouse effect — a cause-and-effect relationship the IPCC calls “very likely”[[Solomon, S., D. Qin, M. Manning, R.B. Alley, T. Berntsen, N.L. Bindoff, Z. Chen, A. Chidthaisong, J.M. Gregory, G.C. Hegerl, M. Heimann, B. Hewitson, B.J. Hoskins, F. Joos, J. Jouzel, V. Kattsov, U. Lohmann, T. Matsuno, M. Molina, N. Nicholls, J. Overpeck, G. Raga, V. Ramaswamy, J. Ren, M. Rusticucci, R. Somerville, T.F. Stocker, P. Whetton, R.A. Wood and D. Wratt, 2007: Technical Summary. (PDF) In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. p. 39.]]. (In its Third Assessment Report, published in 2001, the IPCC called the relationship merely “likely,” which points to the fact that more evidence has accumulated over time).

The rise in temperatures, in turn, has led to a wide range of secondary effects, including rising sea level; the melting of glaciers, mountain snowpack and permafrost worldwide; a decrease in sea ice, especially in the Arctic, during summer; an increase in droughts, wildfires, extreme weather events and insect infestations; changing migration patterns among birds; changes in growing zones for plants; and many more.[[Many of these effects are summarized in Table 9.4 (page 729) of Hegerl, G.C., F. W. Zwiers, P. Braconnot, N.P. Gillett, Y. Luo, J.A. Marengo Orsini, N. Nicholls, J.E. Penner and P.A. Stott: Understanding and Attributing Climate Change. (PDF) In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. 2007 (PDF)]]

While IPCC reports are considered the gold standard in climate-change science, other reports are taken seriously as well; among them is a June, 2009 document from the U.S. Global Change Research Program, which focuses on the regional impacts of climate change within the United States.[[The full report, or smaller, regional reports]]